Thawing rates

Figure 7.5 shows examples of the time course of the potato specimen center and the air temperatures during one, two, and four Ifeeze/thaw cycles performed at a high freezing rate (2°C min ) and a low thawing rate (0.5°Cmin ). The structural rigidity of samples subjected to four freeze/thaw cycles was less than half that of samples subjected to only one cycle (Alvarez and... 

Alvarez, M. D., Canet, W., Fernandez, C. (2007a). Effect of modified starch concentration and freezing and thawing rates on properties of mashed potatoes (cv. Kennebec). J. Sci. FoodAgric., 87,1108-1122. 

Gonzalez-Sanguinetti, S., M. C. Anon, and A. Calvelo. 1985. Effect of thawing rate on the exudate production of frozen beef. Journal of Food Science 50 697-700, 706. 

Ngapo, T. M., I. H. Babare, J. Reynolds, and R. F. Mawson. 1999. Freezing and thawing rate effects on drip loss from samples of pork. Meat Science 53 149-158. 

Cao E, Chen Y, Cni Z, et al. (2003). Effect of freezing and thawing rates on denatnr-ation of proteins in aqneous solntions. Biotechnol. Bioeng. 82 684-690. 

Fig. 4 Effect of the thawing rate on (a) the shear modulus and (b) the fusion temperature of PVA cryogels prepared by freezing of aqueous polymer solutions at —20 °C for 18 h. PVA concentrations were 120 (i), 100 (2), and 80 g/L (3). (Plotted based on data from [44])...

Fig. 22 Optical micrographs at two different magnifications of thin sections of foamed PVA cryogels prepared from the fluid foam produced by whipping of aqueous polymer solutions in the (a, b) absence and (c, d) presence of the surfactant CTAB at a concentration of 0.415 mM. Initial polymer concentration was 120 g/L. Cryotropic gelation conditions fieezing temperature —20 °C freezing duration 18 h, thawing rate 0.03 °C/min. (From [192] with permission from Springer)...

The rate of the thermal cycles has been shown to affect the mechanical properties of the PVA hydrogel. A stiffer material is produced by using a slower thawing rate because of the increased amount of time for the reorganization of polymer chains and squeezing out of water molecules. Hatakeyama et al. showed that the rate of freezing affects the size of the crystals and, therefore, the number of crystals formed [33]. Slower thawing increases the period in which the specimen is at temperatures optimal for gel network formation [13]. 

Lozinsky et al. found that the more time that the solution spends at temperatures below 0 °C, the more time is available for movement of polymer chains. This allows for more time for entanglements to occur and increases the crystallinity by increasing both the number and size of crystallites [34]. This produces PVA-C with increased tensile strength. The rate of freezing has less of an effect on the properties, but has been shown to affect the formation of ice crystals [33]. Thawing rates, on the other hand, affect the formation of the PVA-C and its mechanical properties [7, 18, 35, 36]. 

Fig. 6 Release profiles of BSA from PVA hydrogels subjected to 1 filled circle), 2 (open circle), 3 (filled triangle), and 6 (open triangle) FTCs. These hydrogels were composed of 10 % (w/w) PVA, 0.10 % (w/w) BSA. The freezing and thawing rates were both at 0.50 °C/min...

Fig. 8 Release profiles of BSA fi om PVA hydrogels composed of varying concentrations of PVA at 8 (open circle), 10 (filled triangle), and 15 % (w/w) (filled circle). The hydrogels contained 0.50 % (w/w) BSA and were subjected to two FTCs at a constant freezing and thawing rate of 0.10 °C/min...

Fig. 9 (a) Release profile of Serp-1 from PVA-C in a buffer medium. PVA-C samples were prepared using 10 % PVA solution, 0.1 °C/min freezing and thawing rate, two FTCs, and 200 pg Serp-1. (b) Release profile of Serp-1 from PVA-C inhuman whole blood medium. PVA-C samples were prepared using 10 % PVA solution, 0.1 °C/min Ifeezing and thawing rate, two FTCs, and 200 pg Serp-1. Reprinted from [65] with permission... 

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